Cathode ray tubes, especially those types used in color television and related display applications, are often provided with magnetic shielding means to protect the tube from the deleterious effects of numerous stray voltages, currents, and magnetic fields, including the earth's magnetic field, which tend to adversely influence the desired performance of the tube. While shielding means externally surrounding the frontal portion of the tube have been extensively utilized, it has been found that magnetic shields formed for internal spaced positioning within the tube envelope provide more efficient shielding effects.
A typical internal shielding member conventionally evidences a substantially continuous bowl-like sidewall having a substantially full frontal opening facing the screen region of the tube and a smaller rear-oriented opening. The front portion of the shield is usually affixed to and supported by the frame structure of the shadow mask which is oriented adjacent the screen of the tube. The discretely curved shaping of the shielding member permits close spacing thereof within the encompassing forward region of the funnel portion of the tube envelope. Being so oriented in close adjacency to the funnel, the shield is beneficially demagnetized or neutralized by a degaussing field generated by conventional degaussing means positioned externally of the tube.
In accordance with the state of the art, pressed glass envelope funnels are commonly used in tube manufacture, but some tubes are fabricated with spun glass funnels, sometimes referred to as spin-to-ring funnels. This latter denoted funnel evidences a unique structure in important areas. By the spinning technique of manufacture, the centrifugal movement of material usually produces a substantially continuous built-up strengthening ridge of accumulated glass around the interior surface of the funnel.
Using a conventional internal shielding member, of the type described, with a spun glass funnel necessitates having the sidewall of the shield spaced inward from the interiorly protruding ridge of glass evidenced in the wall of the funnel. This constructional relationship results in an over-all wider than desired shield-to-funnel spacing, which presents the possibility of at least two detrimental conditions. Namely, the shield-to-funnel spatial relationship can be of a width to permit the uncontrolled passage of electron beam overscan therethrough to detrimentally impinge the edge area of the screen. Secondly, as a result of the necessitated wide spacing, the sidewall of the shielding member is further removed from the beneficial effects of the externally initated degaussing field.